Speed control mechanisms for electrical power tools often employ a switch mounted in the handle of the power tool that may be operated by the user to vary the speed of the power tool drive motor. Such switches generally comprise a trigger, or button member, which may be operated by the index finger of the hand of the user holding the power tool. Initial depression of that trigger operates to close switch contacts to connect the device to an electrical source. Further and subsequent depression of the trigger operates a variable resistor to change the amount of resistance present in a speed control circuit, thereby regulating the speed of the power tool drive motor. Such triggers are commonly employed in conjunction with radial arm saws, jigsaws, power drills, power screwdrivers, etc.
Different speeds of operation are often desirable depending upon the application to which the power tool is being used. It may be desirable to operate the power tool a number of times consecutively at a predetermined speed, or it may be desirable to operate the tool at a predetermined constant speed for an extended period of time.
For example, in the case of a radial arm saw or a power drill, a particular speed may be desirable depending upon the material being cut to obtain cleaner cuts in the case of a radial arm saw, or to obtain cleaner bores in the case of a power drill. Also, a particular speed may be desired in order to efficiently and effectively use variously configured or sized appliances, such as different blades or bits. A particular speed may likewise be desired in order to use a power tool for various applications, such as for using a power drill, power screwdriver, sander, nut driver, etc.
The use of a trigger, by itself, is not generally desirable to achieve an optimum operating speed. If the tool is to be used a number of times consecutively, a user is typically unable to depress the trigger to the same position each time to achieve the same desired optimum operating speed. In the case of the power tool being operated at a constant speed for extended periods of time, it is difficult, if not impossible, for a user to hold and maintain a trigger in the same depressed position during such operation and the speed of operation during usage will vary. In the case of a radial arm saw or power drill, the result is that the cut or bore is not as smooth or precise as it could be.
Attempts have been made in the past to provide a speed control mechanism which would enable the user to operate a power tool at a desired optimum speed. However, none of those devices suggest the mechanism disclosed and claimed in this specification. Several of those patents are briefly reviewed here to show the state of the art.
For example, U.S. Pat. No. 2,525,839 issued on Oct. 17, 1950 to Sparklin discloses a "Variable-Speed Electric Drill" and U.S. Pat. No. 3,383,943 issued on May 21, 1968 to Piber discloses a "All-Speed Lever Lock." Both of these patents teach the use of a locking mechanism to lock the trigger in any depressed position. Those mechanisms, however, leave certain problems unsolved. First, if the tool is to be used a number of times consecutively, there is no way to return to the same trigger position each time. Second, these mechanisms do not allow an operator to easily change the speed during operation of the power tool. Still further, the locking device must be released before the trigger position is changed and then must be reset again afterwards.
U.S. Pat. No. 3,221,192 issued on Nov. 30, 1965 to Franklin discloses a "Variable Speed Hand Tool." The mechanism disclosed includes a power tool casing having both an on-off trigger switch and a separate control knob for controlling the speed of the power tool. While this mechanism allows the speed of the power tool to be adjusted during operation, it eliminates the normal function of the trigger, i.e. to vary the operating speed of the power tool. During some operations, a user may want to immediately decelerate the tool depending on varying cutting conditions. That is not possible with the Franklin mechanism without turning the separate control knob in which case the knob would then have to be reset in order to re-achieve the optimum operating speed.
U.S. Pat. No. 3,309,484 issued on Mar. 14, 1967 to Frenzel discloses a "Trigger Actuated Switch Device." This patent teaches the use of an adjustment member mounted rotatably within a trigger and threadably engaging a lug means which abuts the casing of the power tool, thereby limiting the depression of the trigger. The adjustment member has a knob located on the trigger itself which can be turned to move the lug and thereby adjust the extent to which the trigger can be depressed and the maximum speed at which the power tool may be operated. The disadvantage of this mechanism is that adjustments in the operating speed of the power tool cannot be easily and quickly made during operation. First, the index finger operating the trigger partially covers and interferes with rotation of the adjustment member. Thus, the trigger would have to be released and the adjustment member reset each time a change in optimum speed is desired.
Accordingly, there remains a need for a simple mechanism by which the maximum optimum operating speed of a power tool may be adjusted quickly, even during operation of the power tool.